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We clear the clipboard when entering LockedFullscreen.
LockedFullscreen is a state entered to start a secure test or poll.

We clear the clipboard in these cases to prevent cheating,
so it also makes sense to clear ClipboardHistory.

In the future we would like to create a new ClipboardHistory and
restore the old one  after leaving LockedFullscreen, but for now we
will just clear ClipboardHistory.

Bug: 1145651
Change-Id: Id5583567375384f2b2bc877f77acf6c054825977
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521230
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#824930}